The penis bone or ‘baculum’ is common to lots of placental mammals but by no means all of them. It seems to have evolved independently nine times in different mammal lineages but it has also subsequently been lost in many cases. Among primates, humans are the only ones without a baculum, although it is tiny in gorillas and chimpanzees.

The baculum allows prolonged penetration and it is normally only present in animals that mate for longer than three minutes. Lengthy sex sessions are an adaptation to maximise the male’s chance of impregnating the female. Humans evolved monogamy as a reproductive strategy, which – along with other social rules – reduces the risk of females mating with rival males. Men can therefore get away with shorter copulation times.

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